Among the hydrocarbon raw materials or charges which may be chosen within the scope of the invention, there will be noted, besides methane, natural gases, liquefied petroleum gases (LPG), light gasolines and naphthas, heavy fractions of petroleum oil or those originating from its refining, crude oil and, more generally, gaseous and liquid hydrocarbons obtained either from fossil raw materials, schists and tar sands, coal, or from biomass, from matter of animal origin or from various wastes (household refuse, etc.).
As for the basic products which are obtained by conversion of these raw materials and which are at the present time widely employed in the chemical industry, these are either unsaturated hydrocarbons of low molecular weight, such as acetylene, olefins and diolefins, or saturated hydrocarbons, also of relatively low molecular weight, or else possibly carbon monoxide or hydrogen.
Various types of processes and of apparatus for converting hydrocarbons are in existence at present.
First of all, what are known as "thermal" processes may be mentioned. These processes may be divided into two large categories.
The first is that where the energy needed for the conversion reaction is contributed by a high-power electrical source (electric arc or plasma).
In the second category, the energy is of chemical origin and is obtained by the combustion of a part of the hydrocarbon charge itself (processes known by the name of "single-stage") or of another fuel, it being possible for the combustion products obtained to be then either mixed with the hydrocarbon (so-called "two-stage" processes), or used to heat refractory materials fitted to a wall of the reaction chamber, so as to ensure the conversion of this same hydrocarbon (so-called "cyclic" process).
The two essential characteristics of the known single-stage processes consist, on the one hand, in preheating the materials which are to be reacted (namely at least oxygen and a type of hydrocarbon) with the aid of an auxiliary combustion reaction and, on the other hand, in premixing these same materials before the exothermic oxidation reaction which will lead to the conversion of the initial hydrocarbon.
In these single-stage processes, the premixing of the materials, in a chamber provided for this purpose, is considered to be indispensable, the rate of diffusion of the gases being considered incompatible with the contact time of the materials with each other, which is necessarily limited to ensure the conversion reaction in appropriate conditions.
When the progress of reactions in a process of this type is examined more closely, it is noted that the hydrocarbon undergoes, partly, a conversion (to acetylene when methane is involved) and, partly, a conversion to carbon monoxide, hydrogen, carbon dioxide and water. Soot formation is unavoidable in practice and constitutes a disadvantage which is difficult to overcome. The necessary final quenching of the resultant reaction mixture makes it possible, when it is performed in oil, to recover a significant part of the sensible heat of the gases after reaction, while stabilizing the conversion products obtained. On the other hand, the selectivity (that is to say the percentage of hydrocarbons to be converted which are actually converted into the desired hydrocarbon conversion products) is rather mediocre, that is to say that a relatively high proportion of carbon monoxide is produced.
In two-stage processes, the first stage consists in producing a gas at a very high temperature by means of a combustion reaction of the hydrogen-oxygen type. Steam is generally added so as to moderate the temperature rise. In the second stage, the hot gas obtained is mixed with the hydrocarbon which is to be converted and which then undergoes a pyrolysis reaction leading to the pyrolysis products such as acetylene and ethylene in the case where methane has been taken as the initial hydrocarbon.
In general, in order to improve the mixing of the reactant products, the pyrolysis products obtained are circulated through a venturi tube or similar, after which they are quenched.
In practice, a process of this type appears, nevertheless, to be relatively poorly adapted, especially in the case where methane is employed as starting hydrocarbon. Furthermore, it has been found necessary for the walls of the apparatus in which the reaction takes place to be made of highly temperature-resistant materials (refractory materials) in view of the particularly exothermic and violent nature of the reaction.